Heat pump systems for residential and commercial applications often include independent auxiliary heating sources to provide heat when the system load is greater than can be satisfied by operation of the heat pump alone. Heat pump systems are often provided with fossil fuel auxiliary heating sources or furnaces, although electric resistance grid heaters or other sources may also serve as auxiliary or backup heaters. Typically, heat pump systems with auxiliary heat sources are operated such that the fossil fuel or electric backup heater and the heat pump are not operated at the same time to avoid placing excess thermal stresses on the heat pump part of the system.
Moreover, conventional heat pump systems are usually manufactured and shipped to the end user with a controller, including a thermostat. If a fossil fuel or electric resistance heater is installed later, an additional control device and associated wiring must be provided to modify control of the system to allow the heat pump and the auxiliary heat source or furnace to work together. It is desirable to eliminate such an additional control device, the wiring associated therewith and the effort to install the device when a heat pump system is modified to include an auxiliary or backup heat source, such as a fossil fuel furnace, electric resistance heater or other source of auxiliary heat.
Moreover, in certain instances the auxiliary or backup source of heat may fail to operate properly. With conventional prior art systems, the heat pump is not called on to restart to provide whatever heat it is capable of providing. Thus, in such prior art systems there is complete failure to provide heat, even though the heat pump portion of the system is operable to provide at least enough heat to prevent substantial discomfort to the occupants of the space to be heated or prevent a more catastrophic event. It is to overcome deficiencies of prior art heat pump systems with auxiliary heat sources that the present invention has been developed.